The term “oil shale” refers to a marlstone deposit interspersed with an organic mixture of complex chemical compounds collectively referred to as “kerogen.” The inorganic marlstone consists of laminated sedimentary rock containing mainly clay with fine sand, calcite, dolomite, and iron compounds. When the oil shale is heated to about 250–400° F., destructive distillation of the kerogen occurs to produce products in the form of oil, gas, and residual carbon. The hydrocarbonaceous products resulting from the destructive distillation of the kerogen have uses which are similar to petroleum products. Indeed, oil shale is considered to be one of the primary sources for producing liquid fuels and natural gas to supplement and augment those fuels currently produced from petroleum sources.
Processes for recovering hydrocarbonaceous products from oil ‘shale may generally be divided into in situ processes and above-ground processes. In situ processes involve treating oil shale which is still in the ground in order to remove the kerogen, while above-ground processes require removing the oil shale from the ground through mining procedures and then subsequently retorting the oil shale in above-ground retort equipment. Clearly, in situ processes are economically desirable since removal of the oil shale from the ground is often expensive. However, in situ processes are generally not as efficient as above-ground processes in terms of total product recovery.
Historically, prior art in situ processes have generally only been concerned with recovering products from oil shale which comes to the surface of the ground; thus, prior art processes have typically not been capable of recovering products from oil shale located at great depths below the ground surface. For example, typical prior art in situ processes generally only treat oil shale which is 100 feet or less below the ground surface. However, many oil shale deposits extend far beyond the 100 foot depth level; in fact, oil shale deposits of 3000 feet or more deep are not uncommon.
Moreover, many, if not most, prior art processes are directed towards recovering products from what is known as the “mahogany” layer of the oil shale. The mahogany layer is the richest zone of the oil shale bed, having a Fischer assay of about twenty-five gallons per ton (25 gal/ton) or greater. Although the mahogany layer is typically only about four feet thick, this layer has often been the only portion of the oil shale bed to which many prior art processes have been applied.
For economic reasons, it has been found generally uneconomical in the prior art to recover products from any other area of the oil shale bed than the mahogany zone.
Thus, there exists a relatively untapped resource of oil shale, especially deep-lying oil shale and oil shale outside of the mahogany zone, which have not been treated by prior art processes mainly due to the absence of an economically viable method for recovering products from such oil shale.
Another important disadvantage of many, if not most prior art in situ oil shale processes is that expensive rubilization procedures are necessary before treating the oil shale. Rubilization of the in situ oil shale formation is typically accomplished by triggering underground explosions so as to break up the oil shale formation. In such prior art process, it has been necessary to rubilize the oil shale formation so as to provide a substantial reduction in the particle size of the oil shale. By reducing the particle size, the surface area of the oil shale treated is increased, in an attempt to recover a more substantial portion of products therefrom. However, rubilization procedures are expensive, time-consuming, and often cause the ground surface to recede so as to significantly destroy the structural integrity of the underground formation and the terrain supported thereby. This destruction of the structural integrity of the ground and surrounding terrain is a source of great environmental concern.
Rubilization of the oil shale in prior art in situ processes has a further disadvantage. Upon destructive distillation of the kerogen in the oil shale to produce various hydrocarbonaceous products, these products seek the path of lease resistance when escaping through the marlstone of the oil shale formation. By rubilizing the oil shale formation, many different paths of escape are created for the products; the result is that it is difficult to predict the path which the products will follow. This, of course, is important in terms of withdrawing the products from the rubilized oil shale formation so as to enable maximum recovery of the products. Since the products have numerous possible escape paths to follow within the rubilized oil shale formation, the task of recovering the products is greatly complicated.
Other significant problems encountered in many prior art in situ processes for recovering products from oil shale stem from problems in controlling the combustion front established within the oil shale bed which pyrolyzes the kerogen. Typically, a hole is formed within the oil shale bed and a burner is inserted into the hole to provide a burning combustion front for pyrolyzing the kerogen.
Disadvantageously, each hole requires its own burner, which significantly increases the costs of the process. Moreover, if the hole is not straight, problems are encountered in inserting the burner down the hole. Further, it is extremely difficult, if not impossible, to use such burners to heat oil shale which is deeper than a few hundred feet below the ground surface.
Perhaps most importantly, the burning combustion fronts established by the burners in these processes are generally difficult to control since the burners are underground, thereby making it difficult to accurately measure the operation conditions and thus to optimize those conditions by controlling the burners. For example, it is difficult to control or measure the amount of oxygen which must be supplied to the underground burners in order to support the burning combustion fronts; the result is poor stoichiometric control.
It is also difficult to control or accurately measure the temperature of the burning combustion front. Since radiation heat from such underground burners typically results in uneven heating of the oil shale formation, hot and cold spots within the oil shale are often experienced.
The result of such underground burner systems is a poorly controlled and economically inefficient system for pyrolyzing the kerogen and recovering a substantial portion of the products from the oil shale.
Thus, from the foregoing, it will be appreciated that it would be a significant advancement in the art to provide a process and system for recovering hydrocarbonaceous products from an in situ oil shale formation at any depth, and in particular, at depths of up to 3000 feet or greater. Additionally, it would be a significant advancement in the art to recover products from regions of in situ oil shale formations which prior art processes have been economically incapable of treating. Moreover, it would be a significant advancement in the art to provide a process and system for recovering hydrocarbonaceous products from an in situ oil shale formation wherein expensive and time-consuming rubilization procedures are eliminated, in order to preserve the structural integrity of the ground and surrounding terrain, and to eliminate the creation of numerous escape paths for the hydrocarbonaceous products, thereby making the flow path of the products more predictable so as to maximize recovery of the hydrocarbonaceous products. Further, the reduction of maintenance costs accrued by placing burner mechanisms above-ground would provide a significant advantage. Finally, it would be a significant advancement in the art to provide a process and system for recovering hydrocarbonaceous products from an in situ oil shale formation wherein the problems of burning combustion fronts within the oil shale formation, produced by underground burners or other means, are eliminated.